(1) Field of the Invention
The present application relates to an over-current protection device and a protective circuit module (PCM) containing the same, more specifically, to an over-current protection device with high bonding strength to combine with external leads and a protective circuit module containing the same.
(2) Description of the Related Art
Because the resistance of a conductive composite material having positive temperature coefficient (PTC) characteristic is very sensitive to temperature variation, it can be used as the material for current sensing devices, and has been widely applied to over-current protection devices or circuit devices. The resistance of the PTC conductive composite material remains extremely low at a normal temperature, so that the circuit or cell can operate normally. However, when an over-current or an over-temperature event occurs in the circuit or cell, the crystalline polymer of the PTC conductive composite material will melt and expand to sever a lot of conductive paths and therefore the resistance instantaneously increases to a high resistance state (i.e., trip) to diminish the current.
As shown in FIG. 8, U.S. Pat. No. 6,713,210 discloses a protective circuit module with over-current protection function. An IC device 2 is disposed on a protective circuit module 1, and a PTC device 3 is surface-mounted onto a surface of the protective circuit module 1. The PTC device 3 is a laminated structure, in which a PTC material layer 6 is sandwiched between nickel foils or nickel-plated copper foils 7 and 7′. The nickel foils 7 and 7′ serve as electrodes of the PTC material layer 6. A nickel plate 4 serving as an external electrode is secured on the upper surface of the nickel foil 7, and a copper electrode 5 is soldered to the lower surface of the nickel foil 7′ that is adjacent to the surface of the protective circuit module 1. The nickel plate 4 and the copper plate 5 are symmetrical with reference to the PTC device 3. The nickel plate 4 has an end extending out of the PTC device 3 so as to connect to an apparatus such as a battery. When connecting to a battery, the nickel plate 4 may need to bend to conform to the position or shape of the battery, and therefore stress would generate in the nickel plate 4. If the bonding strength of the nickel plate 4 and the PTC device 3 is insufficient, the nickel plate 4 may peel off the PTC device 3.
Because of high voltage and high current in spot-welding process, the PTC device 3 cannot be subjected to spot-welding directly. U.S. Pat. No. 7,852,192 discloses that an insulating layer and an electrode layer are further added to the surface of the PTC device and conductive blind holes are used to electrically connect to the electrode layer and the nickel foil of the PTC device, thereby the device can be subjected to spot-welding directly.
However, in the aforementioned designs, when the external nickel plate electrode is jointed to the device by reflow, solder paste may be daubed unevenly and the thickness of the solder paste may not be well-controlled. As a result, the bonding strength of the nickel plate is not enough. When the PTC device (chip size) becomes smaller, the amount of the solder paste is hard to be accurately controlled and solder paste is easily overflowed because the gap between the PTC device and the nickel plate is hard to keep consistent. Therefore, the bonding strength of the external lead decreases or changes, resulting in low or unstable production yield.